Amorphous Silicon Nanoparticles Research Articles

High Carbon Doping of Amorphous Silicon Nanoparticles in a One-Step Gas-Phase Process for Increased Cycling Stability

Silicon as an anode material for Lithium-Ion-Batteries (LIB) is a promising candidate to increase overall specific energy of batteries in comparison to state-of-the-art graphite anodes. Yet the cycling stability of silicon-based electrodes is still an issue, mainly due to problems related to the swelling of silicon during lithiation to up to 300 % of its original volume. While a breakage of silicon particles can be prevented by choosing a nano-material this has the detrimental effect of increasing the surface exposed to the electrolyte, i.e. reducing coulombic efficiency. Other solutions like coatings of silicon particles and complex electrode structures are shown to increase cycling stability, but are typically connected to higher manufacturing cost.By doping silicon nanoparticles already during production in a gas-phase one-step process, this study investigates the influence of high carbon concentrations up to 20 wt.%. This causes the silicon to stay amorphous, even at elevated temperatures. Furthermore it is shown, that the disturbance of the lattice by carbon atoms helps mitigating stress during lithiation, and can form a less reactive surface. By varying precursor concentrations during material synthesis, a range of materials were produced and analyzed physicochemically and electrochemically. For carbon values up to 15 wt.% our results show a strong increase in cycling stability of up to 80 % capacity retention after 500 cycles, and first cycle coulombic efficiencies of 80 – 90 %. Yet the increased stability is going along with a decrease in specific capacities to 1000 – 1500 mAh/g. These values were obtained for a 75 wt.% silicon, 15 % PAA, 10% C65 electrode and 1 mg/cm2, tested against lithium at 0.5 C. By linking further electrochemical results to particle properties the effects of the carbon doping are further unravelled. This material engineering offers another promising pathway to increase overall performance of silicon containing LIBs and is industrially viable, due to its inclusion in a scalable one-step material synthesis process. Figure 1

High-Throughput Low Frequency Reactor for Non-Thermal Plasma Synthesis of Amorphous Silicon Nanoparticles

High-Throughput Low Frequency Reactor for Non-Thermal Plasma Synthesis of Amorphous Silicon Nanoparticles

Voltage Hysteresis of Silicon Nanoparticles: Chemo‐Mechanical Particle‐SEI Model

AbstractSilicon is a promising anode material for next‐generation lithium‐ion batteries. However, the volume change and the voltage hysteresis during lithiation and delithiation are two substantial drawbacks to their lifetime and performance. The reason for the voltage hysteresis in amorphous silicon nanoparticles covered by a solid‐electrolyte interphase (SEI) is investigated. Concentration gradients inside the nanoscale silicon cannot produce the massive stresses necessary to cause the reported voltage hysteresis. The chemo‐mechanical model shows that plastic deformation of the stiff, inorganic SEI during lithiation and delithiation reproduces the observed silicon open‐circuit voltage hysteresis. Additionally, the viscous behavior of the SEI explains the difference between the voltage hysteresis observed at low currents and after relaxation. It is concluded that the visco‐elastoplastic behavior of the SEI is the origin of the voltage hysteresis in silicon nanoparticle anodes. Thus, consideration of the SEI mechanics is crucial for further improvements.

Atomic Resolution Tomography on Simulated Amorphous Silicon Nanoparticles.

Atomic Resolution Tomography on Simulated Amorphous Silicon Nanoparticles.

Amorphous Silicon Particles/Polyaniline Composites for Hybrid Photovoltaic Solar Cell: an Experimental Feasibility Study

Hybrid heterojunctions of Polyaniline/Amorphous Silicon (PAni / a-Si:H) were synthesized in order to fabricate the active layer of the polymeric solar cells. For this purpose, amorphous silicon nanoparticles were blended with polyaniline which was synthesized through oxidative polymerization. Then the resultant nanocomposite was applied on FTO in the FTO/ZnO/PANI:a-Si/Ag structure using the spin coating method. The effect of amorphous silicon content on light-harvesting efficiency was studied using the UV-VIS spectroscopy data, electrochemical impedance spectroscopy (EIS), and the incident photon to current efficiency (IPCE) analysis. PAni / a-Si:H nanocomposites were characterized structurally and morphologically using Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD) analysis. Current-Voltage measurements of the photovoltaic cells based on PAni / a-Si:H nanocomposites show that increasing the amount of a-Si:H in the active layer up to 40%Wt, the efficiency of the cell increases 2k times compared to the cell made with pure PAni. Also, in general, cell efficiencies increase slightly with decreasing the size of amorphous silicon nanoparticles at a constant PAni / a-Si:H weight ratio. The impact of a-Si:H weight percent is more significant in the case of smaller silicon particles.

Low temperature oxidation of amorphous silicon nanoparticles

The oxide layer is important for the processing and applications of silicon nanoparticles as it provides chemical stability to the material. Herein, the thermal oxidation of sub-10-nm crystalline and amorphous silicon particles in dry air is investigated with nonisothermal kinetic analysis including Kissinger and Flynn-Wall-Ozawa methods at various heating rates. The results indicate that amorphous silicon nanoparticles have a considerably lower activation energy for oxidation (98 kJ/mol) compared to their crystalline counterparts (182 kJ/mol). In situ heating diffuse reflectance infrared Fourier transform spectroscopy was employed to monitor the evolution of surface species of Si nanoparticles during oxidation. The results suggest that backbond oxidation of higher hydrides contribute to the low onset temperature for oxidation of amorphous silicon nanoparticles. Besides, surface ${\mathrm{O}}_{3}\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}$ and Si-OH species likely prevent the formation of a dense silicon oxide layer, resulting in the low temperature oxidation behavior of amorphous silicon nanoparticles. These results provide insight into kinetics and chemistry of amorphous silicon oxidation in the thin film regime.

Effect of Si-Based Anode Lithiation on Charging Characteristics of All-Solid-State Lithium-Ion Battery

The description of the design, manufacturing technology, and test results of thin-film solid-state lithium-ion batteries with a nanocomposite negative electrode Si@O@Al is given herein. This electrochemical system features the hike on the charging curve plateau, which is interpreted as the change from I–V of the Ti-Si@O@Al contact. The latter is due to the change in the type of silicon conductivity during lithiation, as a result of which the ohmic metal-semiconductor contact proves to be biased in the reverse direction, and the charging current is maintained by minority charge carriers. It is shown that the current-conducting component Si@O@Al is formed by a solid solution a-Si(Al), which has a p-type conductivity. The change in the type of conductivity occurs as a result of silicon compensation through lithiation. It was found that Si@O@Al is nonlinear conductor, which can be considered as a percolation cluster formed by amorphous silicon nanoparticles and molecular clusters of silicon dioxide. The height of the Schottky barrier of the Ti|a-Si(Al) contact and the electron affinity of the a-Si(Al) solid solution were estimated.

Energetic characteristics of hydrogenated amorphous silicon nanoparticles

• Plasma-synthesized amorphous Si NPs ( a -nSi) incorporate abundant hydrogen. • The reactivity of a -nSi/KClO 4 increases by a factor of six compared with nAl. • The energetic performance of nSi is correlated with its hydrogen content. • In situ release of hydrogen from hydrides enhances the reactivity. In this work, we utilize a low-temperature non-equilibrium plasma to nucleate and grow sub-10 nm Si particles with varying degrees of crystallinity by tuning the level of power coupled into plasma discharge. Temperature-programmed desorption spectroscopy shows that as-prepared amorphous Si nanoparticles ( a -nSi) incorporate more hydrogen than crystalline Si nanoparticles ( c -nSi). Further, hydrogen is incorporated in the material in the form of higher silicon hydrides with a lower desorption temperature. Combustion cell measurements show that when formulated with KClO 4 , a -nSi outperforms its crystalline counterpart with respect to pressure output. The pressurization rate of a -nSi/KClO 4 composite increases by a factor of six compared with nAl to 0.66 MPa·μs −1 . Evidence of hydrogen release (∼730 K) from Temperature-jump time-of-flight mass spectrometry (T-Jump TOFMS) of a -nSi/KClO 4 suggests the creation of Si dangling bonds prior to ignition (∼820 K), which then react exothermically with oxygen liberated from KClO 4 , leading to ignition. Explosive reaction of H 2 /O 2 mixture likely contributes to the rapid pressure rise. The enhanced energetic performance of hydrogenated a -nSi compared to its crystalline counterpart suggests that incorporation of hydrogen is a promising strategy for improving the performance of nanoenergetic materials.

Formation and quantitative analysis of internal structure of Si nanoparticles developed via bead-milling

Amorphous silicon nanomaterial is isotropic on the macroscale and can effectively inhibit the expansion/contraction during lithiation/delithiation processes, which remarkably improves the cycle performance of Li-ion batteries. Bead-milling is a simple, cost-effective, and scalable method for manufacturing amorphous and/or crystalline silicon nanoparticles. In this work, the internal structure of Si nanoparticles prepared by bead-milling was found to consist of amorphous and nanocrystalline silicon as well as amorphous silicon oxide. X-ray diffraction patterns and Raman spectra are used to calculate the average crystallite size and estimate the degree of crystallization and amorphization of silicon. The quantitative analysis of amorphous silicon oxide is carried out through x-ray photoelectron spectroscopy characterization and oxygen content measuring. It was found that the average particle size (D50) and the crystallite size were reduced to 91 and 3.7 nm, respectively, from 4.06 μm and 50.6 nm before bead-milling, and the degree of amorphization and oxygen content increased to 85.7% and 7.38%, respectively, from 37.5% and 0.12% before bead-milling. It is demonstrated that the longer the milling time, the smaller the sizes of particles and crystals and the higher the ratio of the amorphous phase. However, it inversely causes side-effects such as the increase in oxidization of Si nanoparticles and the increase in content of ZrO2 impurity.

Formation Mechanism of Amorphous Silicon Nanoparticles Synthesized by Induction Thermal Plasma

This study focus on the synthesis of amorphous silicon nanoparticles and understanding the formation mechanism. Counter-flow quenching gases with different flow rates were injected from downstream of the torch to understand the effect of quenching gas on the formation of silicon nanoparticles. Transmission electron microscopy show that nanoparticles with spherical shape and agglomerates consist of smaller particles were synthesized. X-ray diffraction analysis is used to calculate the amorphization degree, which is defined as fraction of amorphous silicon in the silicon nanoparticles including both crystal and amorphous. The obtained results show that higher quenching gas flow rate leads to smaller diameter with higher amorphization degree. Electron diffraction patterns reveal that nanoparticles with diameter less than 10 nm are amorphous and agglomerated together, while for the nanoparticles with diameter larger than 10 nm are crystal. The formation mechanism of amorphous silicon nanoparticles is explained by estimated nucleation temperature and experimental results. Consequently, silicon nucleates at about 2400 K and then silicon vapor condenses on the nucleus. Finally, smaller nanoparticles will keep amorphous phase, while nanoparticles with a larger diameter grow to form crystalline.

Band-Gap Tunability in Partially Amorphous Silicon Nanoparticles Using Single-Dot Correlative Microscopy.

Silicon nanoparticles (Si-NPs) represent one of many types of nanomaterials, where the origin of emission is difficult to assess due to a complex interplay between the core and surface chemistry. Band-gap tunability in Si-NPs is predicted to span from the infrared to the ultraviolet spectral range, which is rarely observed in practice. In this work, we directly assess the size dependence of the optical band gap using a single-dot correlative microscopy tool, where the size of the individual NPs is measured using atomic force microscopy (AFM) and the optical band gap is evaluated from single-dot photoluminescence measured on the very same NPs. We analyze 2–8 nm alkyl-capped Si-NPs prepared by a sol–gel method, followed by annealing at 1300 °C. Surprisingly, we find that the optical band gap is given by the amorphous shell, as evidenced by the convergence of the optical band gap size dependence toward the amorphous Si band gap of ∼1.56 eV. We propose that the structural disorder might be the reason behind the often reported limited emission tunability from various Si-NPs in the literature. We believe that our message points toward a pressing need for development and broader use of such direct correlative single-dot microscopy methods to avoid possible misinterpretations that could arise from attempts to recover size–band gap relation from ensemble methods, as practiced nowadays.

Formation mechanism of amorphous silicon nanoparticles with additional counter-flow quenching gas by induction thermal plasma

The fabrication process of amorphous silicon nanoparticles by induction thermal plasma was studied by experiments and numerical simulations. Additional quenching gas was introduced as counter-flow to the plasma tail flame to enhance quenching effect. The flow rate of quenching gas ranged from 0 to 70 L/min. Amorphous silicon nanoparticles were confirmed by electronic diffraction analysis with random shapes and serious agglomerate, while the crystal particles had a totally different morphology of spherical and freestanding. The quenching rate was estimated on the basis of numerical results and increased from 3.2 × 104 to 8.9 × 105 K/s with quenching gas flow rate. The amount of amorphous silicon increased as quenching gas injection and should be attributed to the improved preparation of small nanoparticles (<5 nm). The above results suggested the formation of amorphous silicon in thermal plasma is controllable.

High thermally stable hybrid materials based on amorphous porous silicon nanoparticles and imidazolium-based ionic liquids: Structural and chemical analysis

Compared to conventional solvents, ionic liquids (ILs) are highly recognized for their ability to enhance the dispersion of nanoparticles (NPs). However, the thermal stability of the ILs-based nanocomposites is a vital parameter for their processing applications. Here, we scrutinized the thermal stability of a series of different imidazolium ion-based ILs before and after incorporating amorphous porous silicon (ap-Si) NPs. The results show that regardless of the obtained quality dispersion, the thermal stability of the host ILs was never regressed. The combination of ap-Si NPs and bmim-SCN (1-buthyl-3-methyl imidazolium thiocyanate) induced highly dispersed framework with an enhanced thermal stability (∼15 °C shift to higher temperature). Likewise, the emim-BF 4 (1-ethyl-3-methylimidazolium tetrafluoroborate) coated the ap-Si NPs forming a very stable dispersion along with a good thermal stability (∼8 °C shift). On the other hand, the thermal stability of bmim-Ac (1-buthyl-3-methylimidazolium acetate) was not affected owing to the high viscosity of bmim-Ac that limited the dispersion of ap-Si NPs at room temperature. Throughout our study, we explored the intermolecular interactions using SEM, TEM, Raman spectroscopy and XRD. We probed the thermal stability of the fabricated dispersions using TGA, and DSC as part of characterization methodology.

Morphology engineering of silicon nanoparticles for better performance in Li-ion battery anodes.

Amorphous silicon nanoparticles were synthesized through pyrolysis of silane gas at temperatures ranging from 575 to 675 °C. According to the used temperature and silane concentration, two distinct types of particles can be obtained: at 625 °C, spherical particles with smooth surface and a low degree of aggregation, but at a higher temperature (650 °C) and lower silane concentration, particles with extremely rough surfaces and high degree of aggregation are found. This demonstrates the importance of the synthesis temperature on the morphology of silicon particles. The two types of silicon nanoparticles were subsequently used as active materials in a lithium half cell configuration, using LiPF6 in an alkylcarbonate-based electrolyte, in order to investigate the impact of the particles morphology on the cycling performances of silicon anode material. The difference in morphology of the particles resulted in different volume expansions, which impacts the solid electrolyte interface (SEI) formation and, as a consequence, the lifetime of the electrode. Half-cells fabricated from spherical particles demonstrated almost 70% capacity retention for over 300 cycles, while the cells made from the rough, aggregated particles showed a sharp decrease in capacity after the 20th cycle. The cycling results underline the importance of Si particle engineering and its influence on the lifetime of Si-based materials.

Outlooks for development of silicon nanoparticle memory cells

Phase change memory is based on changes in the optical, electrical or other properties of materials during phase transitions, e.g. an amorphous to crystalline transition. Currently existing and potential applications of this memory are primarily based on multicomponent alloys of metals and semiconductors. However single-component nanoparticles including Si ones are also of interest as promising nanosized memory cells. The potential for developing this type of memory cells is confirmed by the fact that the optical absorption index of bulk amorphous silicon is of the same order of magnitude as that of crystalline silicon. Certainly this phenomenon can hardly be implemented with a single nanoparticle the size of which is within light wavelength. Using molecular dynamics and the Stillinger-Weber potential we have studied the regularities of melting and the conditions of crystallization of silicon nanoparticles containing within 105 atoms. We have shown that cooling of nanosized silicon drops at a 0.2 TK/s rate or higher rates causes their amorphous transition whereas single-component nanosized metallic drops crystallize in molecular dynamics experiments even at a 1 TK/s rate. Further heating of amorphous silicon nanoparticles containing above 5 ∙ 104 atoms causes their crystallization in a specific temperature range from 1300 to 1400 K. We have concluded that there is a possibility of developing phase change memory cells on the basis of the above phase transitions. An amorphous transition of a nanoparticle can be achieved by its melting and further cooling to room temperature at a 0.2 TK/s rate whereas a crystalline transition, by its heating to 1300–1400 K at a 0.2 TK/s rate followed by cooling. Results of molecular dynamics experiments suggest there is a minimum silicon nanoparticle size for which the development of phase change memory cells becomes theoretically impossible at a given temperature change rate. For a 0.2 TK/s temperature change rate this minimum size is 12.4 nm (number of atoms approx. 5 ∙ 104).

On the prospect of creating memory elements based on silicon nanoparticles

Phase-change memory is based on a change in the optical, electrical, or other properties of a substance during a phase transition, for example, transition from the amorphous to the crystalline state. Already realized and potential applications of such memory are associated with the use for this purpose of multicomponent alloys based on metals, semiconductors. However, single-component nanoparticles, including Si ones, are also of interest in view of the prospects for their use as nanoscale memory units. In particular, possibility of creating such memory units is confirmed by the fact that the bulk phase of the amorphous silicon has an optical absorption coefficient which is by an order of magnitude greater than that of the crystalline, although, it is difficult to release this effect for an individual nanoparticle whose size does not exceed the wavelength of light. In this work, using molecular dynamics (MD) and the Stillinger-Weber potential, we studied the laws of melting and conditions of crystallization for silicon nanoparticles containing up to 100,000 atoms. It has been shown that upon cooling a silicon nanodroplet at a rate of 0.2 TK/s and higher rates, its transition into the amorphous state takes place, whereas single-component metal nanodroplets crystallize even at cooling rates of 1 TK/s. Upon subsequent heating of amorphous silicon nanoparticles containing more than 50,000 atoms, they crystallize in the definite temperature range 1300—1400 K. It is concluded that it is principally possible to create memory units based on the above phase transitions. The transition of a nanoparticle to the amorphous state is achieved by its melting and subsequent cooling to the room temperature at a rate of 0.2 TK/s, and switching to the crystalline state is achieved by heating it to 1300—1400 K at a rate of 0.2 TK/s and subsequent cooling. On the basis of results of MD experiments, a conclusion is made that there exist a minimal size of silicon nanoparticles, for which producing memory units based on the change of the phase state, is not possible. It was found that for the temperature change rate of 0.2 TK/s, the minimal size in question 12.4 nm that corresponds to 50,000 atoms.

Research of silicon and silver nanoparticles obtained by laser ablation in liquid

Laser ablations of silicon and silver in ethanol were made and the nanoparticles obtained were studied. It was shown the silver nanoparticles had both monocrystal and polycrystalline structures and cubic and hexagonal structures. It was cleared the amorphous silicon nanoparticles included a little quantity of the monocrystalline silicon nanoparticles.

Photoluminescence comparison of SRO-LPCVD films deposited on quartz, polysilicon and silicon substrates

In the present work, photoluminescence spectra of silicon-rich oxide monolayers (SRO10 and SRO25) and bilayers (SRO25/10, SRO10/25) films deposited on quartz, polysilicon on quartz and silicon substrates are compared. Silicon-rich oxide films were deposited using Low Pressure Chemical Vapor Deposition (LPCVD) technique. The films were characterized by Ellipsometry, Fourier Transform Infrared Spectroscopy, Secondary Ion Mass Spectrometry, Optical Transmittance, Photoluminescence, Scanning Electron Microscope, and High-Resolution Transmission Electron Microscopy. The results show that the bilayer films, deposited on polysilicon film on quartz have the stronger photoluminescence, which is indicative that there are different types of defects, Silicon nanocrystals, Silicon nanoparticles, and amorphous silicon nanoparticles that improve the Photoluminescence response. Also, it was observed constructive interferences in the transmittance a spectrum of silicon-rich oxide films/polysilicon films on quartz, due to that their refractive indices are different and they are together. This, when a beam of light crosses the interface between materials with different refraction indexes, its direction of propagation is altered, and the bigger is the difference of its refractive indexes, the greater will be the beam refraction. In all materials known this phenomenon is called as positive refraction. The photoluminescence is related to silicon dioxide defects such as weak oxygen bonds, neutral oxygen vacancy, non-bridging oxygen hole centers, or positively charged oxygen vacancies. Moreover, it is a combined effect where the bandgap energy acquires some direct bandgap properties due to quantum confinement, both effects (defects and quantum confinement) are important to increase the photoluminescence intensity. The average diameters of Silicon nanocrystals were estimated from the band gap energy and also by High-Resolution Transmission Electron Microscopy obtaining an average diameter of 3.8 ± 0.08 nm. Comparative analysis of optical and structural properties was performed on all samples.

Solar harvesting based on perfect absorbing all-dielectric nanoresonators on a mirror.

The high-index all-dielectric nanoantenna system is a platform recently used for multiple applications, from metalenses to light management. These systems usually exhibit low absorption/scattering ratios and are not efficient photon harvesters. Nevertheless, by exploiting far-field interference, all-dielectric nanostructures can be engineered to achieve near-perfect absorption in specific wavelength ranges. Here, we propose - based on electrodynamics simulations - that a metasurface composed of an array of hydrogenated amorphous silicon nanoparticles on a mirror can achieve nearly complete light absorption close to the bandgap. We apply this concept to a realistic device, predicting a boost of optical performance of thin-film solar cells made of such nanostructures. In the proposed device, high-index dielectric nanoparticles act not only as nanoatennas able to concentrate light but also as the solar cell active medium, contacted at its top and bottom by transparent electrodes. By optimization of the exact geometrical parameters, we predict a system that could achieve initial conversion efficiency values well beyond 9% - using only the equivalent of a 75-nm thick active material. The device absorption enhancement is 50% compared to an unstructured device in the 400 nm - 550 nm range and more than 300% in the 650 nm - 700 nm spectral region. We demonstrate that such large values are related to the metasurface properties and to the perfect absorption mechanism.

Effect of the Substrate Nature on the Structure and Optical Properties of Amorphous Silicon Nanoparticles Embedded into Porous Anodic Aluminum Template

Effect of the Substrate Nature on the Structure and Optical Properties of Amorphous Silicon Nanoparticles Embedded into Porous Anodic Aluminum Template